There is a continuing need to improve on systems for detecting a fire in its early stages. In particular, today's high technology companies have created a growing number of operational environments within which a significant loss can result from even a small fire. Other areas, such as group homes and hotels, where extensive egress time is necessary due to lack of mobility of the occupants or unfamiliarity of the occupants with their surroundings, can make the speed of fire detection a question of life or death.
To date, the fastest detectors employ continuous sampling type detection systems which use a sampling tube network to draw air samples from an area to be protected into the detection device. By using these networked sampling tubes, a single detector may replace a number of point type detectors. However, because the sampling tube networks carry air samples from a number of different locations to the sensor, it is imperative that networks be designed to obtain predictable performance.
The two primary performance characteristics of a smoke detection system are sensitivity and response time. In a continuous sampling type smoke detection system, the sensor threshold is the smoke density which must enter the sensor unit to cause actuation of the detector. However, this is not equal to the smoke density which must enter a sampling orifice in the sampling network since the smoke sample is diluted while entering a sampling orifice by clean air entering other orifices outside the smoky area. The smoke density sampled at an orifice must therefore be greater than the threshold of the sensor in order to provide a sufficient density at the detector to cause actuation. The smoke density which must be present in the protected area to cause actuation of the sensor, taking into account these dilution effects, is termed the alarm threshold, and defines the sensitivity of the sampling orifice.
The response time of each sampling orifice is defined as the time it takes an air sample entering a sampling orifice to travel to the sensor. In a multi-orifice sampling tube, the response time from each sampling orifice decreases along the sampling tube in the direction of the sensor due to 1) the shorter distance of travel and 2) the increase in air flow rate in the system as each sampling orifice is passed.
Both the sensitivity and response time of each sampling orifice are dependent upon the rate of sampling at that orifice. When equal sized orifices are placed along the sampling tube, the rate of sampling decreases along the sampling tube in the direction away from the sensor, due to the loss of suction pressure in the sampling tube caused by friction. Equal diameter sampling orifices, therefore, do not sample at equal rates and overall performance of the system is unbalanced. Because the system is unbalanced, air enters different sampling orifices at different rates, and therefore smoke entering different orifices undergo unequal dilution effects. Thus, different smoke concentrations in different protected zones are required to achieve an alarm threshold at the sensor.